1. Field of the Invention
The present invention relates to an image reading apparatus and an image forming apparatus.
2. Description of the Related Art
In a known image reading apparatus called a sheet-through type, a document is conveyed in a sub-scanning direction and caused to pass through a fixed reading position to perform the sub-scanning of an image of the document. At the reading position of an image reading unit, a transmitting glass is provided to allow the transmission of light emitted from a light source inside the image reading unit to the outside document and light reflected at the image surface of the document and traveling to the inside of the image reading unit.
If the image reading apparatus of this type is configured such that the document passes through the transmitting glass while coming in contact with the same at reading an image of the document, there is a likelihood that glue, correction fluid, or the like attached to the document is transferred onto the transmitting glass. The transfer of foreign matter such as glue onto the transmitting glass as described above gives an influence on a read image signal as vertical streaks, which results in the degradation of the read image.
Japanese Patent No. 4618157 describes an image reading apparatus in which a sheet member serving as a step member is adhered to the upper surface of a transmitting glass on an upstream side in the conveying direction of a document to provide the step between the transmitting glass and a sheet-member surface serving as a document guiding surface and the document is conveyed with a prescribed distance from the transmitting glass at a reading position.
In addition, Japanese Patent Application Laid-open No. 2004-260246 describes an image reading apparatus in which the end of a sheet member on a downstream side in the conveying direction of a document is made into an inclined surface that gradually gets closer to a transmitting glass toward the downstream side.
If the document is conveyed with a prescribed distance from the transmitting glass as in the image reading apparatus described in Japanese Patent No. 4618157, foreign matter such as glue can be prevented from being transferred onto the transmitting glass at the reading position and the degradation of a read image can be prevented.
In the image reading apparatus described in Japanese Patent No. 4618157, however, the trailing end of the document forcefully moves to the side of the transmitting glass due to the stiffness of the document, i.e., due to the rigidity of the document or the like when passing through the end of the sheet member serving as the step on the downstream side in the conveying direction of the document and then contacts the transmitting glass. As a result, the distance between the document and the reading apparatus suddenly reduces at the reading position. When the distance between the document and the reading apparatus reduces, an image of the document is read in a greater size. This results in a problem in which the size of the image suddenly increases at a certain point of the trailing end of the read image. Particularly, the thickness of a vertical line extending in a sub-scanning direction suddenly increases at a certain point of the trailing end, which makes the degradation of the image conspicuous.
If the end of the sheet member on the downstream side in the conveying direction of the document is made into the inclined surface as in the image reading apparatus described in Japanese Patent Application Laid-open No. 2004-260246, the trailing end of the document can gradually move closer to the transmitting glass as being guided by the inclined surface. Thus, since the size of a read image at the trailing end thereof gradually increases, the degradation of the image can be made inconspicuous compared with a case in which the size of the image suddenly increases.
However, the step (thickness of the sheet member) is about 0.5 mm, and thus it is difficult to form the inclination described above. In addition, FIG. 5 in Japanese Patent Application Laid-open No. 2004-260246 illustrates a configuration in which a plurality of sheet members is superimposed one on another to form steps at the end of a step member on the downstream side in the conveying direction of the document. However, if the sheet members are thick, the document falls from a high position correspondingly. Therefore, the problem in which the size of an image is suddenly changed cannot be substantially prevented. In view of this, it may also be possible that thin sheet members are superimposed one on another to lower the steps of the step-like portion at the end thereof on the downstream side in the conveying direction of the document. In this case, however, the plurality of thin sheet members is required to be bonded together in their deviated state, which gives rise to a problem that a manufacturing cost increases.
Moreover, it may be possible that the step member is made of an elastic member and the step member on the downstream side in the conveying direction of the document is compressed and elastically deformed by the stiffness of the document to prevent the document from falling from a high position. However, according to a configuration in which the step member on the downstream side in the conveying direction of the document is brought into contact with the transmitting glass and compressed and deformed by the stiffness of the document, there is a limit in the compressive deformation. Therefore, the document-falling problem cannot be substantially prevented.
In view of the above, there is a need to provide an image reading apparatus and an image forming apparatus capable of making a change in the size of a read image at the trailing end thereof inconspicuous with a simple configuration.